In a typical power distribution application, power from a central source is distributed through a number of branch circuits to a load device. The branch circuits are equipped with protection devices such as circuit breakers or fuses. During an electrical fault, such as a short circuit, the protection devices are designed to detect an abnormally high level of current and disconnect, or interrupt, the source from the load before causing damage or fire to the distribution system.
The introduction of the Ground Fault Interrupter (GFI) added electrocution protection to the distribution system by detecting an imbalance between phase currents in a particular branch circuit, indicating that current is flowing through an alternate ground path and possibly in the process of electrocuting an individual.
However, there are significant shortcomings in traditional distribution protection methods. For example, a fire could still occur from a loose connection. In this case, the resistance of a live connection increases and heats up to the point of igniting surrounding materials. This heat build-up could occur at electrical currents well below the trip point of the branch circuit protection devices. In the case of GFI protection, the GFI circuit can only protect an individual that comes in contact with both a line conductor and a ground point, such as would be the case if an individual touched a live electric conductor with one hand and a sink faucet with the other hand. However, if the individual manages to touch both a live conductor and a return path (such as across the “hot” and neutral conductors of a home outlet) the GFI would not activate and the person would receive a shock.
Another concept key to the background of the invention of this disclosure is a metric used to relate the lethality of an electric shock to the duration and magnitude of a current pulse flowing through the body. One metric used to describe this relationship by electrophysiologists is known as the chronaxie; a concept similar to what engineers refer to as the system time constant. Electrophysiologists determine a nerve's chronaxie by finding the minimal amount of electrical current that triggers a nerve cell using a long pulse. In successive tests, the pulse is shortened. A briefer pulse of the same current is less likely to trigger the nerve. The chronaxie is defined as the minimum stimulus length to trigger a cell at twice the current determined from that first very long pulse. A pulse length below the chronaxie for a given current will not trigger a nerve cell. The invention of this disclosure takes advantage of the chronoxie principle to keep the magnitude and duration of the energy packet to be safely below the level that could cause Electrocution.
Electrocution is the induction of a cardiac arrest by electrical shock due to ventricular fibrillation (VF). VF is the disruption of the normal rhythms of the heart. Death can occur when beating of the heart becomes erratic, and blood flow becomes minimal or stops completely. McDaniel et. Al. in the paper “Cardiac Safety of Neuromuscular Incapacitating Defensive Devices”, Pacing and Clinical Electrophysiology, January 2005, Volume 28, Number 1, provides a conservative reference for estimating the minimum electrical charge necessary to induce VF under conditions similar to those of the disclosed invention. The study was performed to investigate the safety aspects of electrical neuromuscular incapacitation devices commonly used by law enforcement agencies for incapacitating violent suspects. McDaniel measured the response of a series of pigs to multiple, brief (150 μs) electrical pulses applied to the thorax of the animals. In these tests, a threshold charge of 720 μC could induce VF in a 30 kg animal. The barbed darts were placed on the surface of the animal in close proximity to the heart and penetrated enough to bypass the normal insulating barrier of the skin. This results in a body resistance as low as 400 Ohms. In comparison, the U.S. Occupational Safety and Health Agency (OSHA) describes the resistance of wet human skin to be approximately 1000 Ohms.
By comparing the amount of electrical energy contained in a packet sent by a source to the amount received by the load, it can be determined if some other mechanism, such as an external short circuit, or person receiving a shock, has affected the transfer of energy. The transfer can then be interrupted to protect the equipment or personnel. If the period of a current pulse is below the muscle chronaxie, human skeletal or heart muscles will be much less affected by the pulse. The avoidance of a building or equipment fire is also critical, but the level of energy to cause a fire is normally much less than that which would cause cardiac arrest. The disclosed invention monitors and controls these small packets of energy, and thus offers additional safety over what can be provided even by the combination of a circuit breaker and a ground fault interrupter.
There are two primary fault modes that must be detected. The first mode is an in-line or series fault where an abnormal resistance is put in series with the path between the source and load as is illustrated by the individual being shocked in FIG. 4a. The second fault mode is a cross-line or parallel fault as is illustrated in FIG. 4b. The in-line fault can be detected by an abnormal drop in voltage between the source and load points for a given electrical current. The cross line fault is detected by a loss in current between the source and load, due to the shunting of current through the parallel fault element. Both the in-line and cross-line fault settings will require compensation for normal line resistances and leakage currents to avoid false or nuisance shutdown of the power distribution system.